DE102013204964B4 - Optically pumped surface emitting lasers with high reflectivity reflector and limited bandwidth - Google Patents

Abstract

A laser structure comprising:a semiconductor gain region configured to emit radiation at a lasing center wavelength;an optical pump source configured to emit a pump beam (110) at a pump center wavelength directed toward the gain region; a first reflector comprising a distributed Bragg reflector (DBR) and positioned in the pump beam, the first reflector having a reflectivity greater than 90% over a bandwidth of less than 60 nm centered at the laser centroid wavelength, the first reflector also has a reflectivity of less than 50% at the pump center wavelength;a second reflector, wherein the gain region is disposed between the first reflector and the second reflector,wherein the first reflector comprises a number of layer pairs, each layer pair having a first and a second layer comprising an optical thick e of the first layer is 3/4 the lasing centroid wavelength, and an optical thickness of the second layer is 3/4 the lasing centroid wavelength, and wherein the layer pairs of the first reflector comprise dielectric material.

Description

US 2011/0 268 143 A1 describes a light-emitting semiconductor device containing a pumping light source, an amplification structure and an output coupling mirror. The gain structure consists of InGaN layers that exhibit resonant excitation absorption at the pump wavelength. Light from the pump light source causes the gain structure to emit light, which is reflected back to the gain structure by the output mirror. A distributed Bragg reflector causes internal reflection within the gain structure. The output mirror allows light of sufficient energy to pass through for use outside of the device. A frequency doubling structure can be arranged between the amplification structure and the output coupling mirror. Output wavelengths in the deep UV spectrum can be achieved.

U.S. 6,553,051 B1 describes an optical assembly that includes an optical subassembly that includes a prefabricated long wavelength laser optically coupled to a prefabricated short wavelength laser located in a housing. The optics assembly can be removably installed in the housing in which the short wavelength laser is contained. The short wavelength laser optically pumps the long wavelength laser, resulting in long wavelength laser power. The optical assembly enables the independent manufacture, optimization and testing of the short wavelength laser and the long wavelength laser.

US 2007/0 019 697 A1 describes a VCSEL system comprising forming a first mirror, forming a vertical cavity on the first mirror, the vertical cavity integrating multiple gain regions, and forming a transverse p/n junction lateral to the integrated Multiple gain regions comprised where the forward biased transverse p/n junction causes photon emission in the integrated multiple gain regions.

US 2006/0 268 398 A1 describes a MEMS tunable semiconductor optical amplifier (SOA). The device includes a substrate, a first mirror coupled to the substrate, a second mirror, an active region coupled between the first and second mirrors, and a microelectromechanical actuator coupled to the second mirror, wherein a The microelectromechanical actuator is energized to tune the SOA.

US 2007/0 104 241 A1 describes a surface emitting laser. A vertical outer cavity surface emitting laser (VECSEL) using end pumps is provided in which a pump beam is returned using a pump reflective layer to increase pump absorption. The VECSEL includes: an active layer for generating and emitting signal light; an external mirror separated from and facing the top of the active layer and transmitting a first portion of the signal light and reflecting a second portion of the signal light to the active layer; a first reflection layer that contacts a lower surface of the active layer and reflects the signal light to the outside mirror; a pump laser for emitting the pump beam toward the lower surface of the active layer to excite the active layer; and a second reflective layer touching the top of the active layer and reflecting a portion of the pump beam back to the active layer.

US 2006/0251140 A1 describes an end-pumped vertical surface cavity laser (VECSEL) in which a pump laser beam strikes a laser chip at right angles. In the external cavity surface emitting laser, a laser chip package is provided with a laser chip that emits light having a first wavelength by optical pumping. An external mirror is spaced from a top surface of the laser chip package to transmit part of the light emitted from the laser chip to the outside and to reflect the rest to the laser chip, a heat sink is coupled to the bottom surface of the laser chip package to cool the light emitted by the laser chip To dissipate heat generated by the laser chip, and a pumping laser faces a bottom surface of the heatsink and emits pumping light having a second wavelength perpendicular to the laser chip.

US6597017B1 describes a semiconductor device having pseudo-lattice-matched layers with good crystallinity formed with lattice-mismatched materials. Tensionally strained n-type Al0.5Ga0.5N layers (bottom side) and compressively strained n-type Ga0.9In0.1N layers (top side) are grown in 16.5 periods on a GaN crystal layer substrate to form an n- type to form DBR mirror; an undoped GaN spacer layer and an active area are grown on the n-type DBR mirror; and an undoped GaN spacer layer is grown on the active area. Further, p-type Al0.5Ga0.5N tensile layers (lower side) and p-type Ga0.9In0.1N compressive layers (upper side) are grown in 12 periods on the spacer layer to form a p-type DBR mirror and finally complete a surface emitting semiconductor laser.

DE19708992A1 describes a VCSEL.

The VCSEL has a semiconducting substrate with a first split Bragg reflector with alternating layers of InAlGaP and AlAs. These layers are of a dopant type and are concentrated on one of their surfaces. An active area is applied to a coverage area and a second coverage area is applied to the active area. A second Bragg reflector on the second cap region has alternating layers of InAlGaP and AlAs and an opposite doping type and concentration. A contact area is attached to the second reflector.

Summary of the Invention

It is the object of the present invention to improve an optically pumped surface emitting laser. This object is achieved by a laser structure according to claim 1 and a method according to claim 9. Embodiments of the invention are laid down in the dependent claims.

character list

• 1 Figure 13 is a diagram of an optically pumped vertical cavity surface emitting laser (VCSEL) including a high reflectivity/bandwidth limited (HR/BL) distributed Bragg reflector (DBR);
• 2 Figure 12 illustrates a configuration of a VECSEL device where the pump source and the semiconductor structure are arranged such that the pump beam impinges the semiconductor structure at an angle, θ ≠ 0;
• 3 Figure 13 is a diagram of an optically pumped external vertical cavity surface emitting laser (VECSEL) comprising an HR/BL DBR;
• 4 Figure 12 presents simulation plots of reflectivity and transmittance versus wavelength of incident radiation for a HR/BL DBR comprising 8 pairs of SiO ₂ /TiO ₂ deposited on GaN and having three-quarter wavelength layer thicknesses designed for a centroid wavelength of 460 nm are designed;
• 5 provides reflectance and transmittance angular spectrum simulation plots for the three-quarter wavelength HR/BL DBR with SiO ₂ /TiO ₂ layers (as described in conjunction with 4 described) for a pump wavelength of 405 nm;
• 6 provides simulation plots of reflectivity and transmittance for the three-quarter wavelength HR/BL DBR with SiO ₂ /TiO ₂ layers (as in connection with 4 described) for a pump wavelength of 445 nm;
• 7 compares predicted reflectance to measured reflectance from various experimental structures;
• 8th Figure 12 is a diagram of a VECSEL that includes a small air gap between the gain region and the reflector;
• 9 represents the laser spectrum laser radiation emission produced by a first experimental laser structure corresponding to that in 2 VCSEL shown is similar;
• 10 represents the output power of the VCSEL of the first experimental laser structure;
• 11 shows the emission of laser radiation generated by a second experimental laser structure corresponding to that in 8th illustrated VECSEL, which included an air gap;
• 12 Figure 12 depicts a simulation of the resonator modes of a system comprising the moving external mirror, the air gap, the reduced GaN residue and the DBR of the epitaxial side; and
• 13 FIG. 11 illustrates a VCSEL that includes first and second reflectors disposed on either side of a gain region, the first reflector having a first portion, and the thermal conductivity of the first portion is different than the thermal conductivity of the second portion.

DESCRIPTION OF VARIOUS EMBODIMENTS

Vertical-cavity surface-emitting lasers (VCSEL) and external vertical-cavity surface-emitting lasers (VECSEL) (collectively referred to as V(E)CSEL) are of interest because of the high quality of their laser spectral and spatial optical characteristics. The realization of V(E)CSELs within the III-nitride material system for ultraviolet (UV), blue and green emission is difficult. The embodiments discussed herein include configurations based on III-nitride materials for the formation of compact and low-cost V(E)CSEL systems that emit in the near-UV to blue spectral range. In some cases, the V(E)CSEL systems can include frequency doubling elements to reach wavelengths down to the UV-C range.

The embodiments discussed below include novel distributed Bragg reflectors (DBRs) integrated into the optically pumped laser systems. The DBRs discussed herein can be used with any type of semiconductor laser, e.g. B. edge emitters or surface emitters, which are particularly useful VCSEL or VECSEL constructions. The DBR designs discussed herein provide high reflectivity for the desired V(E)CSEL laser wavelength and high transmission for compact and inexpensive semiconductor pump sources. Some designs include DBRs with high reflectivity and limited reflection bandwidth. For example, some DBRs discussed below exhibit greater than 90% reflectivity in a reflection bandwidth of less than about 60 nm at the laser centroid wavelength. These DBRs have a transmission efficiency of more than about 50% at the centroid wavelength of the pump radiation. In some cases, the high reflectivity DBRs, which have a reflection bandwidth limited to near the laser wavelength, are fabricated using three-quarter wavelength thick DBR layers, which significantly reduce the DBR's reflection bandwidth and freedom in pump source selection to allow. The use of a high reflectivity, limited bandwidth (HR/BL) DBR can be helpful, as these DBRs allow commercially available, high-power semiconductor pump sources with fixed emission wavelengths to be integrated into optically pumped laser systems.

In general, high quality mirrors are required for the short gain medium of surface emitting lasers. The DBRs used for such devices can have a reflectivity greater than 90%, or greater than 95%, or even greater than 99% at the lasing wavelength. Optical pumping of V(E)SLs allows more flexibility in DBR materials since the DBR does not need to be electrically conductive and dielectric DBRs can be used. In addition, optical pumping can reduce the absorption of the pump radiation and the generation of the electron-hole pairs predominantly in the gain region, e.g. B. quantum wells, which reduces the threshold conditions for the emission of laser radiation.

wobei λ_l die Schwerpunkt (Spitzen)-Wellenlänge der Laserstrahlung ist, und n(λ_l) der Brechungsindex des Schichtmaterials bei der Wellenlänge der Laserstrahlung ist.To implement optically pumped V(E)CSELs in a compact and low-cost system, the choice of the pump source is of interest. Optical pump sources with a high optical output power (e.g. up to 1 watt) are currently commercially available on the basis of GaN-based semiconductor lasers in the wavelength range of 405 nm and 445 nm. However, quarter-wave dielectric DBRs, which provide high reflectivity with relatively few layers of material, are not optimal for V(E)CSELs using these pump sources, since the quarter-wave DBRs have a relatively wide reflection bandwidth and therefore at wavelengths of 405 nm and 445 nm nm are reflective. Although generally applicable to lasers based on various material systems, the examples discussed below include the construction of DBRs for V(E)CSELs based on III-nitride material systems operating at ultraviolet (UV), blue, and green -Emit wavelengths. The proposed DBRs provide sufficient reflectivity for the desired V(E)CSEL laser wavelength and transmittance for the pump sources. The requirements of high reflectivity, e.g. B. a reflectivity of more than 95%, and limited reflection bandwidth at the laser wavelength in combination with a sufficient transmissivity for the pump radiation wavelength can be achieved using different materials and layer thicknesses. In one implementation, a high reflectivity, limited bandwidth (HR/BL) DBR can be fabricated using three-quarter wavelength thick material layers, which significantly reduce the DBR's reflection bandwidth and allow freedom in pump configuration design. For a three quarter wavelength DBR, the thickness of the i th DBR layer is given by the following equation: $$t_i=3{\lambda }_i/4n\left({\lambda }_i\right)$$

where λ _l is the centroid (peak) wavelength of the laser radiation and n(λ _l ) is the refractive index of the layer material at the wavelength of the laser radiation.

In some implementations, the materials used for the DBR layers are combinations of dielectric materials that have a relatively high refractive index contrast, such as SiO2 and TiO2. The use of these high refractive index contrast materials means that relatively few layers can be used to achieve high reflectivity. It is also possible to use materials with a lower refractive index contrast, although more layers are then needed to achieve high reflectivity. The materials of the DBR layers may include dielectric and/or semiconductor materials, which may be epitaxially grown or non-epitaxially deposited.

1 Figure 12 is a diagram of an optically pumped vertical cavity surface emitting laser (VCSEL) that includes an HR/BL DBR. The VCSEL and VECSEL devices discussed herein can be operated as continuous wave (CW) lasers. The VCSEL of 1 comprises an optical pump source, a first reflector which is the HR/BL-DBR, a second reflector and a semiconductor gain region. As in 1 indicated, the laser optical resonator is defined by the first and second reflectors. The gain region may include multiple quantum well (QW) structures, and each QW structure may include one or more quantum wells. The gain region may be fabricated by epitaxially growing the semiconductor layers of the gain region onto a substrate, which is then reduced to leave a substrate residue 140. FIG. In some cases, the gain region 10 may comprise periods of InGaN double QWs grown by metalorganic vapor phase epitaxy on a GaN substrate. The dual QWs can be arranged in a resonant periodic amplification scheme such that the positions of the QWs are aligned with the electric field pattern of the laser mode. The first reflector (HR/BL-DBR) was applied to the gain region. The GaN substrate was reduced and the second reflector was applied directly to the backside of the gain region.

For example, the gain region may include multiple quantum well structures, such as about 10 periods of InGaN-based double quantum well structures, grown on a substrate. Each quantum well structure may include the following layers in order: an InGaN biasing layer (In0.03Ga0.97N, 35.3 nm thick), a first thin spacer layer (GaN, 5 nm thick), a first quantum well, (In0.18Ga0.82N , 3 nm thick), a second thin spacer layer (GaN, 5 nm thick), a second quantum well, (In0.18Ga0.82N, 3 nm thick), a thick spacer layer (GaN, 21.7 nm thick) and a carrier confinement and stress control layer (AI0.2Ga0.8N, 20 nm thick). Additional structures and methods that may be used in connection with the approaches discussed herein are disclosed in the commonly owned U.S. Patent Application S/N 13/427,105 disclosed.

The pump radiation 110 passes through the first reflector and the gain region and creates electron-hole pairs in or near the quantum wells of the gain region. The electron-hole pairs diffuse into the quantum wells and recombine to produce laser radiation. The laser radiation is reflected by the first and second reflectors within the optical laser resonator, creating a standing wave that provides resonant periodic amplification. Laser radiation of sufficient energy passes through the second reflector as indicated by arrow 130. Some implementations include an optional frequency converter, such as a non-linear optical crystal, that generates radiation at harmonic waves or at sum or difference frequencies of the laser radiation. When using the frequency converter, laser outputs in the deep UV spectrum, e.g. B. less than 280 nm can be achieved. the inside 1 The optional frequency converter shown can optionally be used for all of the embodiments described herein. Frequency doubling can be more efficient if the frequency converter, e.g. B. a non-linear crystal, is arranged within the resonator of a VECSELs, as in 3 shown.

In some embodiments, the pump source is a gallium nitride (GaN) based laser diode (or alternatively a plurality of laser diodes) emitting in the 370 to 530 nm range. For example, commercially available laser devices designed for digital Blu-ray players or projector displays emit at 405 nm or 445 nm. These devices are currently available and can be used as pump sources for GaN-based V(E)CSELs. The output power of these pump sources can range from 0.5 to 10 watts. As in 1 As shown, the pump source optics can be configured to provide a focusing system with one or more lenses that focus the pump radiation to a pump spot size of 50 to 200 µm in diameter to achieve a power density in excess of 50 kW/cm2 . The gain region outputs a beam at a desired wavelength, for example in the 420 to 550 nm range.

During operation, the gain region can become hot. To reduce the possibility of performance degradation or even damage to the device due to excessive heat generation, the device may be mounted on a heat sink with the first reflector in close proximity to the heat sink. For example, the heat sink may be made of copper or another thermally conductive material such as diamond. An optional second heat sink can be placed in close proximity to the second reflector. As in 1 As shown, the heat sink has an aperture to allow pump radiation access to the first reflector and gain region. Or alternatively, in cases where the heat sink is optionally transparent to the pump wavelength, e.g. B. a permeable diamond, no opening needed in the heat sink. When the second heat sink is used in close proximity to the second reflector, the second heat sink also has an opening to allow the laser radiation 130 to be emitted. In some configurations, these openings are centered one above the other. As in 1 1, in some cases the pump beam impinges on the first reflector normal to the surface of the first reflector. In this configuration, the pump source and the semiconductor gain region can be coaxially aligned, allowing a linear arrangement of all optical parts of the structure, as in FIG 1 illustrated.

2 Figure 12 illustrates another configuration for a VCSEL device wherein the pump source and gain region are arranged such that the radiation 110 from the pump source and the radiation 130 output from the gain region are non-coaxial. With this arrangement, the pump beam impinges on the semiconductor structure at an angle, θ ≠ 0. Changing the angle of incidence of the pump beam for a particular wavelength shifts the transmittance/reflectance characteristics of the first reflector, as will be discussed in more detail below.

3 Figure 12 illustrates yet another possible configuration for a laser structure. In this example, the laser is configured as a VECSEL. The implementation of 3 resembles in some respects the in 2 VCSEL configuration shown, except that the VECSEL includes an external resonator 301 separating the second reflector, ie the external drop mirror, from the gain region. The external mirror may have a curved surface to adopt the Gaussian beam profile of the laser mode. The long resonator (e.g. 50 to 200 nm) allows the insertion of additional optical components, such as non-linear crystals for second harmonic generation or polarizing filters to fine-tune the wavelength of the laser emission. In some implementations, the VECSEL substrate on which the gain region is grown can be retained to provide structural support for the VECSEL. The substrate may be of sufficient thickness to facilitate processing of the device, or may be about 100 µm thick, for example. In these implementations, the substrate material would be a wide bandgap material, e.g. B. GaN, or other materials which have a high transmittance for both the pump radiation and the laser radiation. in the in 3 In the configuration shown, the heat sink does not require an opening since the device is pumped from the opposite side of the gain region.

As already mentioned, the first reflector comprises in 1 until 4 a HR/BL DBR which can be fabricated using a variety of materials and layer thicknesses. The materials of the DBR layers may include dielectric and/or semiconductor materials, which may be epitaxially grown or non-epitaxially deposited. In some cases, the HR/BL-DBR may comprise a number of pairs of layers, each layer having a thickness as set forth in Equation 1 above. The number of layer pairs used in the HR/BL-DBR depends on the materials of the layers, but for high refractive index materials as few as 7 layer pairs can be used.

4 Figure 12 illustrates simulation plots of reflectance 410 and transmittance 420 versus wavelength of incident radiation for an HR/BL DBR comprising 8 pairs of SiO ₂ /TiO ₂ deposited on GaN and having layer thicknesses specified for a centroid wavelength of 460 nm are designed. In this example, assuming that η(λl) = 1.46 and 2.2 for SiO2 and TiO2, respectively, each of the SiO2 layers has a thickness of tSiO2 = 3 x 460 nm/4 x 1.46 = 236.30 nm and each of the TiO2 layers has a thickness of tTiO2 = 3 x 460 nm/4 x 2.48 = 139.11 nm. How out 4 As can be seen, the high reflectivity (>99%) can be realized with a fairly narrow bandwidth of about 55 nm versus the about 150 nm for a quarter wavelength SiO2/TiO2 DBR. In addition, 4 to recognize that wavelengths outside this high reflection band, e.g. B. wavelengths from about 400 nm to about 437 nm, have a transmissivity of more than 50%. Several wavelength bands have greater than 90% transmittance. For example, at normal incidence, pump radiation wavelengths of about 435 nm, 423 nm, and 405-410 nm have greater than 90% transmission.

As previously mentioned, two wavelengths (405 nm and 445 nm) of high power InGaN laser diodes are commercially available with optical output powers up to 1 watt. Some examples provided below provide configurations that can be applied to these potential pump radiation wavelengths, although implementation of the approaches described herein are not limited to these specific pump radiation wavelengths.

5 provides simulation plots of reflectance 510 and transmittance 520 for the three-quarter wavelength HR/BL DBR with SiO ₂ /TiO ₂ layers (as described in connection with 4 described) for a wavelength of 405 nm. Out of 5 It can be seen that the DBR provides high transmissivity for angles from 0 to 12, 29 and 40 degrees, making these angles particularly suitable for pumping. 6 provides simulation plots of reflectivity 610 and transmittance 620 for the three-quarter wavelength HR/BL DBR with SiO ₂ /TiO ₂ layers (as described in connection with 4 described) for a wavelength of 445 nm. The angular spectrum of the three-quarter wavelength HR/BL DBR shows that pumping at an angle of 43 degrees provides high transmission at 445 nm.

The simulation results were verified with experimental structures. Three quarter-wavelength HR/BL DBRs were fabricated with target wavelengths ranging from approximately 460 nm to 470 nm. These experimental structures included three-quarter wavelength DBRs with four layer pairs of SiO ₂ /TiO ₂ deposited on quartz and BK7. The results of the experimental structures agreed well with the theoretical prediction, as in 7 you can see. In 7 line 710 represents the reflectivity prediction results, line 720 represents the reflectivity test results of a first 4 layer pair DBR deposited on quartz, line 730 represents the reflectivity test results of a second 4 layer pair DBR deposited on quartz , and line 740 represents test results for the reflectivity of a second 4 layer pair DBR deposited on BK7.

In some configurations, a small air gap can be used between the second reflector and the gain region of a VECSEL structure, as in 8th illustrated to improve the spectral quality of a pump source. The VECSEL of 8th includes a first reflector. In general, the first reflector can include any type of reflector. In some implementations, the first reflector is an HR/BL DBR that has a relatively narrow reflection bandwidth centered at the lasing wavelength, λlase. The narrow reflection bandwidth of the HR/BL DBR does not significantly reflect radiation at the pump wavelength, Apump, allowing the device to be optically pumped by the pump source. The relatively narrow reflection (R) bandwidth of the HR/BL DBR allows the pump radiation to pass through the first reflector and reach the gain region, while the laser radiation generated in the gain region is reflected back to the optical laser resonator. For example, in embodiments using a first HR/BL-DBR reflector, the first reflector may comprise 8 pairs of SiO ₂ /TiO ₂ with layer thicknesses of 236.30 nm for the SiO 2 layers and 139.11 nm for the TiO 2 layers .

In some implementations, the second reflector may have a relatively wide reflection bandwidth with high reflectivity for both the pump radiation wavelength, λpump, and the lasing wavelength, λlase. For example, the second reflector may comprise a quarter-wavelength DBR with 6.5 layer pairs of SiO ₂ /TiO ₂ , the SiO 2 layer having a refractive index, nSiO 2 , of 1.47 and a thickness of 78 nm, and the TiO 2 layers having a refractive index , nTiO2, of 2.2 and a thickness of about 52 nm. This DBR has a reflectivity of greater than 99% in a bandwidth of approximately 150 nm centered at a wavelength of 460 nm.

Providing a small air gap 830 within the optical laser resonator can produce single mode emission of the VECSEL with multimode pump radiation. This embodiment provides a viable approach to improving the spectral quality of a pump source. The air gap 830 located between the second reflector and the gain region forms a third reflective surface 835 in the optical laser resonator at the interface between the air gap 839 and the gain region. The reflective surface 835 and the second reflector define a secondary optical cavity that has a shorter optical length than the laser optical cavity. The secondary optical resonator and the (primary) optical laser resonator form coupled resonators. The coupling between the primary and secondary optical cavities results in certain output modes being favored over others. When a preferred mode overlaps the gain spectrum, single-mode operation becomes possible (see, for example, the simulation given below in 12 is shown). In some embodiments, the distance between the reflective surface 835 and the second reflector can be adjusted until the device emits laser radiation with fewer output modes than input modes of the pump source.

In some implementations, the air gap can be dynamically adjusted to provide one or more selected output modes, e.g. a single dominant output mode. The gain region and/or the second reflector may have electrodes 840 disposed on their inwardly facing surfaces that face each other, with elastically deformable spacer layers 850 between the electrodes 840. Signals 836 may be provided to electrodes 840 by a control system to cause changes in the distances between the inwardly facing reflective surfaces 835 and the second reflector, such as electrostatic, electromagnetic or piezoelectric, and change the shape of the region between them. The air gap distance, or the distance between the reflective surfaces forming the air gap, can have a wide range of values, e.g. B. on the order of about one micron to tens of microns. Larger air gaps allow additional optical components to be inserted into the air gap. For example, non-linear crystals can be inserted in the air gap to generate second harmonics, or polarizing filters can be used to fine-tune the wavelength of the laser emission.

9 represents the laser spectrum laser radiation emission produced by a first experimental laser structure corresponding to that in 2 VCSEL shown, which does not include an air gap. In the first experimental structure, the gain region (10 periods of InGaN double quantum wells arranged in a resonant periodic gain (RPG) scheme) was grown on a bulk GaN substrate. A quarter wavelength SiO2/TiO2 DBR (first reflector) was applied to the gain region. The GaN substrate was reduced to a thickness of less than 100 µm by thin polishing. The second reflector, comprising a quarter-wavelength SiO2/TiO2 DBR, was applied to the polished backside of the substrate. 11 represents the emission of laser radiation generated by a second experimental laser structure corresponding to that in 8th illustrated VECSEL, which included an air gap 830 of about 10 µm between the gain region and the second reflector. As previously mentioned, other air gap thicknesses could be used to achieve a similar result. For the second experimental structure, both reflectors were quarter wavelength SiO2/TiO2 DBRs. Both experimental structures were operated and pumped with a pulsed dye laser emitting at 384 nm with an incidence angle of approximately 40 degrees.

In both the first and second experimental devices, the material quality of the gain chips was confirmed by structural and optical characterization methods. High-resolution X-ray diffraction measurements in combination with transmission electron microscopy studies were used to determine optimal parameters for the structural properties. The growth process was controlled to form sharp layer interfaces and avoid the development of extended defects such as V-type defects. The root mean square (rms) surface roughness of the epitaxial surface of the gain region was set at 0.15 nm for a 2 µm x 2 µm atomic force microscopy (AFM) scan. The internal quantum efficiency (IQE) of the samples was determined by temperature-dependent photoluminescence measurements. The IQE of the laser samples exceeded 50%.

9 Figure 12 shows the laser emission spectrum of the first experimental structure with the emission having a centroid wavelength 910 of 453.7 nm. Several longitudinal modes 920, 921 are clearly visible. The line width of the laser modes is less than 0.1 nm. The mode spacing between the individual laser modes correlates with the length of the resonator (optical resonator), which in this case consists of the thickness of the reduced GaN substrate and the epitaxial layers. The thickness was set at about 73 µm.

10 Figure 12 plots the output power of the VCSEL (first experimental structure) versus the peak pump power of the dye laser at room temperature. The peak pump power was determined by measuring the average pump power impinging on the sample and considering the pulse length and repetition frequency of the pump laser. The threshold pump power can be set to around 0.75W. For an estimated pumping point diameter of about 30 µm, the threshold pumping power was about 100 kW/cm2.

11 shows the laser spectrum of the VECSEL with an air gap between the GaN chip and the DBR mirror (second experimental structure). How out 10 As can be seen, the optical properties of the laser emission changed significantly. While several longitudinal modes were recorded for the first experimental laser structure without an air gap, the spectrum of the second experimental laser structure with the air gap shows a single dominant longitudinal mode 1100. In addition, the laser wavelength has shifted to 440 nm. Both features can be explained by considering the additional interface 835 between semiconductor and air.

12 FIG. 12 depicts a simulation of the resonator modes 1210 of the system including the moving external mirror, the air gap, the reduced GaN residue, and the DBR of the epitaxial side. The lasing wavelength 1220 corresponds to one of these modes. Also shown for comparison is the photoluminescence spectrum 1230 of the sample without the second DBR under exactly the same excitation conditions as for laser operation placed. How out 12 As can be seen, the resonator determines the lasing wavelength, which does not necessarily have to match the photoluminescence emission peak wavelength of the QWs.

Incident pump photons have higher energy than the laser photons, and the energy difference between pump and laser photons is called the quantum error. The energy difference between pump and laser photons is dissipated as heat from the active region of the device. In addition, heat generation from the non-ideal material quality takes place in the form of non-radioactive recombination. Dielectric reflectors can use high refractive index contrast materials that provide very good reflectivity with relatively few layers. However, dielectrics can have relatively low thermal conductivity, and when placed between the gain region and the heat sink, reflectors made from these materials can impede heat transfer from the gain region to the heat sink. In some embodiments, laser structures may include reflectors that have thermal conductivity that varies with distance from the gain region. For example, the thermal conductivity of the reflector may have a thermal conductivity that varies with distance. For example, the thermal conductivity, k1, in a first region of the reflector may be different from the thermal conductivity, k2, in a second region of the reflector. Such reflectors are better suited to achieve two design objectives of acceptably high reflectivity, which can be achieved using high refractive index contrast dielectrics, and acceptably high thermal conductivity, which can be achieved using semiconductor materials that have a higher thermal conductivity than the thermal conductivity of have dielectric materials.

13 FIG. 11 illustrates a VCSEL that includes first and second reflectors disposed on either side of a gain region. In this example, the first reflector is in close proximity to a heat sink and includes first and second sections (in 13 referred to as Section 1 and Section 2 respectively). The first section has a thermal conductivity that is different than the thermal conductivity of the second section.

In some implementations, the first portion may be made from semiconductor materials that have relatively higher thermal conductivity. For example, the first portion of the first reflector may include a semiconductor material epitaxially grown on the gain region. The second section may be made of dielectric materials that have a relatively lower thermal conductivity than the semiconductor material.

The second section of the first reflector can e.g. B. be applied by sputtering or vapor deposition on the first portion of the first reflector.

In some cases, the first section may be an epitaxially grown semiconductor DBR section comprising a number of layer pairs of GaN/AlGaN (thermal conductivity about ksemi=1.3 W/cm-K), or other suitable semiconductor material. The second section may be a non-epitaxial, dielectric DBR section, e.g. Layer pairs of SiO ₂ / TiO ₂ (thermal conductivity about k diel = 0.04 W / cm-K) includes. For example, in some implementations, the first section may include 10.5 layer pairs of GaN/Al0.2Ga0.8N, where the GaN layers have a thickness of about 46.8 nm and the AlGaN layers have a thickness of about 48.8 nm . The second section may comprise 4 layer pairs of SiO2/TiO2, the SiO2 layers having a thickness of about 78.8 nm and the TiO2 layers having a thickness of about 53.2 nm.

If the material with higher thermal conductivity, e.g. B. semiconductors, in conjunction with a material of lower conductivity, z. B. dielectric, is used, the overall thickness of the dielectric portion, which reduces the overall thermal resistance of the device. In some embodiments, one or both of the first section and the second section may be an HR/BL-DBR. The first section of the HR/BL-DBR may be formed from fourth-wavelength GaN/Al-GaN pairs, and the second section of the HR/BL-DBR may be formed from three-quarter-wavelength SiO2/TiO2 layer pairs.

A number of values and ranges are provided in various aspects of the described implementations. These values and ranges are to be treated as examples only and are not intended to limit the scope of the claims. For example, embodiments described in this disclosure may be practiced in any of the disclosed numerical ranges. In addition, a number of materials suitable for different facets of the implementations are identified. These materials are to be treated as examples only and are not intended to limit the scope of the claims.

The foregoing description of various embodiments has been provided for the purpose of illustration Illustrative and descriptive and not limiting. The disclosed embodiments are not intended to be exhaustive or to limit the possible implementations of the disclosed embodiments. Many modifications and variations are possible in light of the above teachings.

Claims (9)

Laser structure comprising: a semiconductor gain region configured to emit radiation at a lasing center wavelength; an optical pump source configured to emit a pump beam (110) directed at a pump center wavelength towards the gain region; a first reflector comprising a distributed Bragg reflector (DBR) and positioned in the pump beam, the first reflector having a reflectivity greater than 90% over a bandwidth of less than 60 nm centered at the laser centroid wavelength, the first reflector also has a reflectivity of less than 50% at the pump center wavelength; a second reflector, wherein the gain region is located between the first reflector and the second reflector, wherein the first reflector comprises a number of layer pairs, each layer pair comprising a first and a second layer, an optical thickness of the first layer being 3/4 the laser centroid wavelength, and an optical thickness of the second layer being 3/4 the lasing centroid wavelength is, and where the layer pairs of the first reflector comprise dielectric material. laser structure claim 1 , where a number of layers is greater than 6. laser structure claim 1 , wherein the first reflector comprises: a first portion having a first thermal conductivity; and a second portion having a second thermal conductivity less than the first thermal conductivity. laser structure claim 1 , wherein the centroid wavelength of the pump radiation is 370 to 530 nm, and the laser centroid wavelength is in a range of 390 to 550 nm. laser structure claim 1 , wherein the pump beam (110) impinges on a surface of the first reflector at an angle with respect to the normal to the surface of the first reflector. laser structure claim 1 , wherein: the semiconductor gain region has a first side and a second side; the first reflector is positioned in close proximity to the first side of the gain region and in the pump beam; a reflective surface is disposed in close proximity to the second side of the gain region; and an air gap is disposed between the second reflector and the reflective surface. laser structure claim 6 , further comprising positioning elements configured to adjust a thickness of the air gap. laser structure claim 6 , wherein the pump beam comprises multiple modes and the laser radiation comprises a single mode. Method comprising: Operating a laser comprising a semiconductor gain region configured to emit radiation at a lasing center wavelength, first and second reflectors, and a reflective surface disposed in close proximity to the gain region, the first and the second reflector define a primary laser cavity and the reflective surface and the second reflector define a secondary optical cavity; operating a pump source configured to emit pump radiation having a number of input modes, the pump radiation being directed toward and at least partially absorbed in the semiconductor gain region; and adjusting a gap between the reflective surface and the second reflector until the laser outputs a number of output modes lower than the number of input modes, wherein the first reflector comprises a number of layer pairs, each layer pair comprising a first and a second layer, an optical thickness of the first layer being 3/4 the laser centroid wavelength, and an optical thickness of the second layer being 3/4 the lasing centroid wavelength is, and where the layer pairs of the first reflector comprise dielectric material.

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